The present invention relates to wear parts and cutting tools manufactured in an economical way from hard materials having smaller contents of hard principles than cemented carbide. In particular, the invention relates to tools consisting of elongated bodies such as shank end mills, broaches, threading tools, drills, shearing and punching tools (e.g., nibbling tools) holding tools such as boring or turning bars, etc. In regard to wear parts, the invention relates essentially to products for rolling mills and transport equipment--in which even mediatransport is included--such as rollers, rolls (e.g., entry guides, transport rolls, etc.) sleeves, bars, shafts and similar products, optionally provided with a center hole, compressor and pump parts, valves etc.
For a long time, it has been desired to make wear parts and cutting tools from material having properties between cemented carbide and high speed steel in an economically satisfactory way. Some such materials exist, as e.g., Ferro-TiC, carbide enriched powder high speed steel, material according to the Swedish Pat. No. 392.482, etc. Economic manufacturing methods have not been realized, however, and said materials have not shown the advantages expected.
Thus, materials such as e.g., Ferro-TiC has not proved successful. This is due to the large grain growth of the hard constituents which takes place during sintering, the high level of cost (being the same as that of cemented carbide because of the same technology) and the high costs of manufacturing.
The so-called particle metallurgical high speed steels can contain a relatively large amount of hard constituents compared to conventional high speed steels, which hard constituents are mainly in the form of vanadium carbide. The amount of hard constituents is limited, however, because of the precipitation of primary carbides from the melt in connection with granulation in inert gas (if there are high contents of vanadium and carbon), because of the machinability since a solid bar is machined with current methods and because of the grindability in making the final tools or wear parts. The particle metallurgical steels are prepared, as mentioned before, by granulation of a melt in inert gas. This process gives a spherical powder, which cannot be compacted to a green body, so the compaction must be done in a container which accompanies the material in the rest of the process. The advantage of the particle metallurgical steels is the low content of oxygen and the small grain size of the hard constituents in the range of 1-2 .mu.m.
Powder metallurgical high speed steel is made via granulation of a melt in water. This process has the same limitation of the alloying content as that of the particle metallurgical steels. Water granulated powder gives good green strength. The powder can thus be used for pressing of shaped bodies which then are sintered to almost final shape. This process makes very great demands upon the sintering furnace and the method has therefore not been used very much. In addition, this process is unsuitable for the production of long, slender tools of the type mentioned above. Also, during sintering, grain growth of the hard constituents, particularly in the grain boundaries is easily obtained. This grain growth will give an insufficient strength in the sintered body.
The practical limit when making cemented carbide is less than 20-25% by weight of binder phase. Even at these levels, there are problems with islands of binder phase formed after the sintering. These islands naturally do not have the hardness of the carbide phase. In the normal manufacture of cemented carbide, the sintering temperature is considerably higher than the temperature at which an alloy consisting of hard constituents (metal carbide particles)+binder phase melts. Consequently, all of the binder phase is melted and it has also dissolved a large amount of the hard constituents. A carbide skeleton remains, however. It is said skeleton which preserves the shape of the body. When there are too large amounts of the binder phase, the carbide skeleton is insufficient and the body loses its shape.
Extrusion is a method of working metallic material which gives possibilities of forming materials relatively difficult to work. The method is advantageously used e.g., in making seamless tubes of high alloyed stainless steel. The drawback of the method is its high cost. In attempts with alloys having extremely high amounts of hard constituents, it has been found that even a tungsten carbide-cobalt alloy having as high an amount of hard constituents as 80% by weight of WC, i.e. cemented carbide, can be warm extruded. Such an alloy has naturally a great resistance to deformation but is normally considered uneconomical to extrude because it causes too great a wear of the extrusion tools. The upper limit is about 25-30% by volume of hard constituents in materials being worked by means of forging, rolling and so on.
It has previously been considered difficult to co-extrude two materials having different resistances to deformation into a compound bar or compound tube. In our attempts to decrease the wear of the extrusion tools it has been found possible, however, to co-extrude a core of normal steel (solid or in powder form) with an outer cover of a powder body being extremely rich in hard particles. It has been found important that this compound body is enclosed in an extrusion can of carbon steel or stainless steel, useful in the very extrusion process and also in the following processes of manufacturing tools or wear parts. The steel core can consist of tool steel or high speed steel.
According to the preceding text, it is possible to extrude bar having up to 70% by volume of hard constituents (80% by weight of WC corresponds to 70% by volume of WC). The hard material according to the present invention relates to alloys in the intermediate range, i.e., 30-70% by volume of hard constituents. The hard constituents consist essentially of carbides and nitrides and the intermediate forms of the metals Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and/or W. Also, hard particles other than carbides and nitrides such as oxides, borides, silicides, etc., may be present. The matrix of the hard material consists of Fe-, Ni- and/or Co-based alloys. Preferably, the matrix of the hard material is based upon iron.
In the manufacture of long, slender tools, such as shank end mills and drills, twisted or straight (axial) flutes are ground in a cylindrical blank. Even at moderate flute depths a long contact curve is formed between the work piece and the grinding wheel. If said contact curve is too long in a material which is difficult to grind, the surface easily burns because the cooling is insufficient and the tendency of smearing is great. The only ways of decreasing the risks of burning is either decrease the removal rate or to use a softer wheel which itself wears quicker and when worn does not maintain the desired profile. The length of the contact curve, b, is about proportional to the source root of .phi..sub.s .multidot.a in which .phi..sub.s is the diameter of the grinding wheel in mm and a is the actual grinding depth. In a normal shank end mill with a diameter of 20 mm, the flute depth is greater than 4 mm which gives a contact curve of about 40 mm. This means very long grinding times in a difficultly ground material if burning is to be avoided. At the same time, we know that in many applications the cutting tool material is used only in peripheral cutters. In those cases where central cutting edges are used, the cutting speed on those edges are lower than that on the outer edges which is why the demands upon wear resistance and toughness for each of these edges also are different.